This invention relates in general to electromagnetic friction clutches and in particular to an improved structure for a rotor for such an electromagnetic friction clutch.
Clutches are well known devices which are commonly used in machinery to selectively connect a source of rotational power to a rotatably driven mechanism. A basic clutch structure includes an input member connected to the source of rotational power, an output member connected to the rotatably driven mechanism, and means for selectively connecting the input member to the output member for concurrent rotation. When the means for selectively connecting is engaged, the input member is connected to the output member so as to rotatably drive the mechanism. When the means for selectively connecting is disengaged, the input member is disconnected from the output member, and the mechanism is not rotatably driven. Many different types of clutches are known in the art for accomplishing this general purpose.
In some clutches, the means for selectively connecting includes a pair of members which are selectively moved into frictional engagement with one another to connect the input member to the output member. Such a friction clutch may include an input member which is fixed in a predetermined axial position and an armature which is constrained to rotate with the output member, but is free to move axially relative thereto between engaged and disengaged positions. In the engaged position, the armature frictionally engages the input member such that the output member is driven to rotate therewith. In the disengaged position, however, the armature is spaced apart from the input member and, therefore, provides no rotational driving connection therewith. Often, the armature is normally maintained in the disengaged position by a resilient spring so as not to frictionally engage the input member unless affirmatively moved into the engaged position by an actuator. Friction clutches of this general type are well known in the art.
In some friction clutches, an electromagnet is used to cause movement of the armature between the engaged and disengaged positions. Electromagnetically actuated friction clutches operate on the principle that a magnetic field which is created about a component formed from a magnetically permeable material will exert a mechanical force on that component. This mechanical force will urge the component to move to a position of minimum resistance relative to the flow of magnetic flux (lines of force) generated by the magnetic field, usually referred to as a position of minimum reluctance. Thus, in electromagnetically actuated friction clutches, the armature and the input member are usually both formed from a magnetically permeable material. When the electromagnet is energized, the electromagnetic field generated thereby attracts the armature toward the input member. As a result, the armature is moved from the disengaged position to the engaged position to connect the input member to the output member and, thus, cause the driven device to be rotatably driven by the source of rotational power.
The input member of a typical electromagnetically actuated friction clutch is embodied as an annular pole piece having a generally U-shaped cross section defined by a radially extending pole face extending between concentric inner and outer tubes. The pole face is normally axially separated from the armature by a relative small air gap. Because they are both formed from a magnetically permeable material, the armature will be attracted to move axially toward the pole face when the electromagnet is energized. To increase the magnitude of this magnetic attraction, and thereby increase the torque transmitting capability of the clutch as a whole, the armature and the pole face are frequently divided into one or more pole regions by a non-magnetically permeable material. These separate pole regions cause the magnetic flux generated by the electromagnet to jump back and forth several times across the air gap separating the armature and the pole face when the electromagnet is energized. For reasons which are well known in the art, this magnetic flux discontinuity structure, or more simply flux break, is effective to increase the magnitude of the magnetic attraction between the armature and the pole piece when the electromagnet is energized.
In the past, both the armature and the pole piece have been formed from a magnetically permeable material, and the flux breaks have been provided by forming slots or recesses in both the armature and the pole piece. These slots or recesses define relatively large air gaps between adjacent pole regions of the magnetically permeable material. Although effective to increase the magnetic attraction between the armature and the pole piece, the formation of such slots or recesses in pole face necessitates the removal of material from the armature or the pole piece, which has been found to weaken the overall strength thereof. Because electromagnetically actuated friction clutches of this type are often used to selectively transmit relatively high torque loads, any weaknesses in the armature or the pole piece undesirably diminish the torque transmitting capacity of the clutch as a whole. Furthermore, because electromagnetically actuated clutches of this type are often used in automotive applications, such as in air conditioner compressor assemblies, they must be relatively simple and inexpensive in construction. Accordingly, it would be desirable to provide an improved component for use in an electromagnetically actuated clutch having one or more flux breaks, such as an armature or a pole piece, and a method for manufacturing same, which addresses these concerns.